Surface acoustic wave filter and communication device using the filter

ABSTRACT

A surface acoustic wave filter has a piezoelectric substrate; at least an input IDT electrod arranged on a piezoelectric substrate; and at least an output IDT electrod arranged on the piezoelectric substrate. A pitch of electrode fingers of the input IDT electrode and a pitch of electrode fingers of the output IDT electrode are different from each other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a longitudinal-mode surface acousticwave filter, a method of manufacturing a surface acoustic wave filter,and a communication device.

2. Related Art of the Invention

In recent years, surface acoustic wave filters have been widely used inmobile communication devices. Surface acoustic wave filters of alongitudinal mode type or a ladder type are used as a filter in a radiofrequency (RF) stage. With the improvement in performance ofcommunication devices such as portable telephones, there has been anincreasing demand for reducing the loss and increasing the attenuationin surface acoustic wave filters.

A conventional longitudinal-mode surface acoustic wave filter will bedescribed.

FIG. 12 shows a configuration of a conventional longitudinal-modesurface acoustic wave filter. As shown in FIG. 12, the surface acousticwave filter has a piezoelectric substrate 801, first, second, and thirdinterdigital transducer (IDT) electrodes 802, 803, and 804, and firstand second reflector electrodes 805 and 806, the IDT electrodes and thereflector electrodes being formed on the substrate. The upper electrodefingers of each of the second and third IDT electrodes 803 and 804 isconnected to an input terminal IN, while the lower electrode fingers ofeach of the second and third IDT electrodes 803 and 804 is grounded. Thelower electrode fingers of the first IDT electrode 802 is connected toan output terminal OUT, while the upper electrode fingers of the firstIDT electrode 802 is grounded. The distances between centers of adjacentpairs of the electrode fingers of the first, second, and third IDTelectrodes 802, 803, and 804, represented by the distance indicated by Pin FIG. 12 (hereinafter referred to as “pitch”), are equal to eachother. The longitudinal-mode surface acoustic wave filter is thusconstructed.

In the above-described surface acoustic wave filter, the electrodefingers are arranged with a constant pitch in order that the acousticvelocity of a surface acoustic wave be constant through the arrangementof the first, second, and third IDT electrodes 802, 803, and 804. Inmany instances, however, the number of electrode fingers of the firstIDT electrode 802 and that of each of the second and third IDTelectrodes 803 and 804 are set different from each other according to adesign considering the bandwidth and impedance. Ordinarily, the surfaceacoustic wave filter is designed so that the number of electrode fingersof the first IDT electrode 802 is larger than that of each of the secondand third IDT electrodes 803 and 804.

A longitudinal-mode surface acoustic wave filter has also been usedwhich is designed so that the electrode fingers of each of electrodeshave different pitches as shown in FIG. 13 to achieve a reduction inloss for example. The conventional longitudinal-mode surface acousticwave filter shown in FIG. 13 is based on a design in which each IDTelectrodes have an electrode finger pitch different from that in a mainregion.

Referring to FIG. 13, the surface acoustic wave filter has apiezoelectric substrate 1201, first, second, and third IDT electrodes1202, 1203, and 1204, and first and second reflector electrodes 1205 and1206, the IDT electrodes and the reflector electrodes being formed onthe substrate. The upper electrode fingers of each of the second andthird IDT electrodes 1203 and 1204 is connected to an input terminal IN,while the lower one of each of the second and third IDT electrodes 1203and 1204 is grounded. The lower electrode fingers of the first IDTelectrode 1202 is connected to an output terminal OUT, while the upperelectrode fingers of the first IDT electrode 1202 is grounded.

Also, referring to FIG. 13, if the pitch in a region indicated by 1 a inthe first IDT electrode 1202 is P, P is ½ wavelength. If the pitch in aregion indicated by 1 b is P′, P′ is smaller than ½ wavelength. Thepitch in a region indicated by 2 a in the second IDT electrode 1203 isP, and P is ½ wavelength. The pitch in a region indicated by 2 b is P′,and P′ is smaller than ½ wavelength. Similarly, the pitch in a regionindicated by 3 a in the third IDT electrode 1204 is P, and P is ½wavelength. The pitch in a region indicated by 3 b is P′, and P′ issmaller than ½ wavelength.

Thus, in each of the first IDT electrode 1202, the second IDT electrode1203, and the third IDT electrode 1204, different electrode fingerpitches are set between the electrode fingers in the same IDTelectrodes.

Also in many instances relating to the arrangement shown in FIG. 13, thenumber of electrode fingers of the first IDT electrodes 1202 and that ofeach of the second and third IDT electrodes 1203 and 1204 are setdifferent from each other according to a design considering thebandwidth and impedance. Ordinarily, the surface acoustic wave filter isdesigned so that the number of electrode fingers of the first IDTelectrode 1202 is larger than that of each of the second and third IDTelectrodes 1203 and 1204.

There is a problem in such a surface acoustic wave filter that there isa limit to improvement in filter characteristics in achieving awide-band characteristic.

SUMMARY OF THE INVENTION

In view of the above-described problem, an object of the presentinvention is to provide a wide-band surface acoustic wave filter havinga steep out of-band attenuation characteristic, a method ofmanufacturing the surface acoustic wave filter, and a communicationdevice.

One aspect of the present invention is a surface acoustic wave filtercomprising:

a piezoelectric substrate;

at least an input IDT electrode arranged on said piezoelectricsubstrate; and

at least an output IDT electrode arranged on said piezoelectricsubstrate,

wherein a pitch of electrode fingers of said input IDT electrode and apitch of electrode fingers of said output IDT electrode are differentfrom each other.

Another aspect of the present invention is the surface acoustic wavefilter, wherein the pitch of electrode fingers of the IDT electrodelarger in number of electrode fingers in said input and output IDTelectrode is larger than the pitch of electrode fingers smaller innumber of electrode fingers.

Still another aspect of the present invention is a surface acoustic wavefilter comprising:

a piezoelectric substrate;

at least an input IDT electrode arranged on said piezoelectricsubstrate; and

at least an output IDT electrode arranged on said piezoelectricsubstrate,

wherein the metalization ratio of said input IDT electrodes and themetalization ratio of said output IDT electrodes are different from eachother.

Yet still another aspect of the present invention is the surfaceacoustic wave filter, wherein the metalization ratio of an IDT electrodelarger in number of electrode fingers in said input and output IDTelectrodes is lower than the metalization ratio of an IDT electrodesmaller in number of electrode fingers.

Still yet another aspect of the present invention is the surfaceacoustic wave filter, wherein if an IDT electrode has a plurality ofelectrode finger pitches, the pitch of main excitation electrode fingersis set as a basic pitch.

A further aspect of the present invention is the surface acoustic wavefilter, wherein a peak frequency of a radiation characteristic of saidinput IDT electrode is substantially equal to a peak frequency of aradiation characteristic of said output IDT electrode.

A still further aspect of the present invention is the surface acousticwave filter, wherein one of said input IDT electrode and said output IDTelectrode comprises a first IDT electrode including a pair of electrodefingers opposed to each other;

the other of said input IDT electrode and said output IDT electrodecomprises a second IDT electrode including a pair of electrode fingersopposed to each other, and a third IDT electrode including a pair ofelectrode fingers opposed to each other, said second IDT electrode beingplaced on one side of said first IDT electrode, said third IDT electrodebeing placed on the other side of said first IDT electrode;

said first, second, and third IDT electrodes are arranged along adirection in which a surface acoustic wave propagates; and

the peak frequency of the radiation characteristic of said first IDTelectrode is substantially equal to the peak frequency of the radiationcharacteristic of each of the second and third IDT electrodes.

A yet further aspect of the present invention is the surface acousticwave filter, wherein one of said input IDT electrode and said output IDTelectrode comprises first, fourth, and fifth IDT electrodes eachincluding a pair of electrode fingers opposed to each other;

the other of said input IDT electrode and said output IDT electrodescomprises a second and third IDT electrodes each including a pair ofelectrode fingers opposed to each other;

said second and third IDT electrodes are placed on opposite sides ofsaid first IDT electrode;

said fourth IDT electrode are placed on the side of said second IDTelectrodes opposite from the side on which said first IDT electrode areplaced;

said fifth IDT electrode are placed on the side of said third IDTelectrode opposite from the side on which said first IDT electrode areplaced;

said first, second, third, forth and fifth IDT electrodes are arrangedalong a direction in which a surface acoustic wave propagates; and

the peak frequencies of the radiation characteristics of at least morethan one of the group of said first IDT electrode, and the group of saidfourth and fifth IDT electrodes, and the group of said second and thirdIDT electrodes are substantially equal to each other.

A still yet further aspect of the present invention is the surfaceacoustic wave filter, wherein the film thickness of said first IDTelectrode and the film thickness of each of said second and third IDTelectrodes are different from each other.

An additional aspect of the present invention is the surface acousticwave filter, wherein the material of said first IDT electrode and thematerial of each of said second and third IDT electrodes are differentfrom each other.

A still additional aspect of the present invention is the surfaceacoustic wave filter, wherein the metalization ratio of said first IDTelectrode and the metalization ratio of each of said second and thirdIDT electrodes are equal to each other;

the number of electrode fingers of said first IDT electrode is largerthan the number of electrode fingers of each of said second and thirdIDT electrodes; and

the electrode finger pitch of said first IDT electrode is larger thanthe electrode finger pitch of each of said second and third IDTelectrodes.

A yet additional aspect of the present invention is the surface acousticwave filter, wherein the metalization ratio of said first IDT electrode,the metalization ratio of said second IDT electrode and the metalizationratio of said third IDT electrode are different from each other.

A still yet additional aspect of the present invention is the surfaceacoustic wave filter, wherein a plurality of filter tracks each havingfirst, second, and third IDT electrodes, and first and second reflectorelectrodes are formed on said piezoelectric substrate, and saidplurality of filter tracks function as one filter in cooperation witheach other.

A supplementary aspect of the present invention is the surface acousticwave filter, wherein each of said plurality of filter tracks isidentical in configuration to the others.

A still supplementary aspect of the present invention is the surfaceacoustic wave filter, wherein at least one of said plurality of filtertracks is different in configuration from the others.

A yet supplementary aspect of the present invention is the surfaceacoustic wave filter, further comprising a first reflector electrodeplaced on the opposite side of said second IDT electrode on saidpiezoelectric substrate opposite from the side on which said first IDTelectrode are placed; and

a second reflector electrode placed on the side of said third IDTelectrode on said piezoelectric substrate opposite from the side onwhich said first IDT electrode are placed,

wherein said first, second, and third IDT electrodes and said first andsecond reflector electrodes are arranged along a direction in which asurface acoustic wave propagates.

A still yet supplementary aspect of the present invention is a method ofmanufacturing a surface acoustic wave filter, comprising

a piezoelectric substrate;

an input IDT electrode arranged on the piezoelectric substrate; and

an output IDT electrode arranged on the piezoelectric substrate,

wherein said method makes a pitch of electrode fingers of said input IDTelectrode and a pitch of electrode fingers of said output IDT electrodedifferent values.

Another aspect of the present invention is a communication devicecomprising:

a transmitting circuit which outputs a transmitted wave; and

a receiving circuit to which a wave to be received is input,

wherein a surface acoustic wave filter is used in said transmittingcircuit and/or in said receiving circuit.

Still another aspect of the present invention is a communication devicecomprising:

a transmitting circuit which outputs a transmitted wave; and

a receiving circuit to which a wave to be received is input,

wherein the surface acoustic wave filter is used in said transmittingcircuit and/or in said receiving circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a configuration of a surface acoustic wave filterin a first embodiment of the present invention.

FIG. 2 is a diagram showing a relationship between (peak frequencies) ofradiation characteristics and the number of electrode fingers.

FIG. 3 is a diagram of a configuration of a surface acoustic wave filterin a second embodiment of the present invention.

FIG. 4 is a graph of radiation characteristics of IDT electrodes in thefirst and third embodiments of the present invention.

FIG. 5 is a graph of a path characteristic of the surface acoustic wavefilter in the first and third embodiments of the present invention.

FIG. 6 is a diagram of a configuration of a surface acoustic wave filterin a third embodiment of the present invention.

FIG. 7 is a diagram of a configuration of a surface acoustic wave filterin a fourth embodiment of the present invention.

FIG. 8(a) is a graph of radiation characteristics of IDT electrodes inthe second embodiment of the present invention.

FIG. 8(b) is a graph of a pass characteristic of the surface acousticwave filter in the second embodiment of the present invention.

FIG. 9(a) is a graph of radiation characteristics of IDT electrodes in aconventional surface acoustic wave filter in which peak frequencies ofradiation characteristic of IDT electrodes do not coincide with eachother.

FIG. 9(b) is a graph of a pass characteristic of the surface acousticwave filter relating to FIG. 9(a).

FIG. 10 is a diagram of a configuration of a surface acoustic wavefilter in a fifth embodiment of the present invention.

FIG. 11 is a diagram of a configuration of a surface acoustic wavefilter in a sixth embodiment of the present invention.

FIG. 12 is a diagram of a configuration of a conventional surfaceacoustic wave filter.

FIG. 13 is a diagram of a configuration of another conventional surfaceacoustic wave filter.

DESCRIPTION OF SYMBOLS

101 piezoelectric substrate

102 first IDT electrodes

103 second IDT electrodes

104 third IDT electrodes

105 reflector electrode

106 reflector electrode

301 piezoelectric substrate

302 first IDT electrodes

303 second IDT electrodes

304 third IDT electrodes

305 reflector electrode

306 reflector electrode

307 filter track

308 fourth IDT electrodes

309 fifth IDT electrodes

310 sixth IDT electrodes

311 reflector electrode

312 reflector electrode

313 filter track

601 piezoelectric substrate

602 first IDT electrodes

603 second IDT electrodes

604 third IDT electrodes

605 reflector electrode

606 reflector electrode

701 piezoelectric substrate

702 first IDT electrodes

703 second IDT electrodes

704 third IDT electrodes

705 reflector electrode

706 reflector electrode

707 filter track

708 fourth IDT electrodes

709 fifth IDT electrodes

710 sixth IDT electrodes

711 reflector electrode

712 reflector electrode

713 filter track

PREFERRED EMBODIMENTS OF THE INVENTION

Embodiments of the present invention will be described with reference tothe drawings.

(First Embodiment)

A first embodiment of the present invention will be described. FIG. 1schematically shows a surface acoustic wave filter which represents afirst embodiment of the present invention.

Referring to FIG. 1, the surface acoustic wave filter has apiezoelectric substrate 101, first, second, and third IDT electrodes102, 103, and 104, and first and second reflector electrodes 105 and106, the IDT electrodes and the reflector electrodes being formed on thesubstrate.

The second IDT electrode 103 and the third IDT electrode 104 are placedon the opposite sides of the first IDT electrode 102. The reflectorelectrode 105 is placed on the side of the second IDT electrode 103opposite from the side on which the first IDT electrode 102 are placed.The reflector electrode 106 is placed on the side of the third IDTelectrode 104 opposite from the side on which the first IDT electrode102 are placed. Thus, the first, second, and third IDT electrode 102,103, and 104 and the first and second reflector electrodes 105 and 106are arranged along the direction of propagation of a surface acousticwave.

The upper electrode fingers of each of the second and third IDTelectrodes 103 and 104 is connected to an input terminal IN, while thelower electrode fingers of each of the second and third IDT electrodes103 and 104 is grounded. The lower electrode fingers of the first IDTelectrode 102 is connected to an output terminal OUT, while the upperelectrode fingers of the first IDT electrode 102 is grounded.

The number of electrode fingers of the first IDT electrode 102 is largerthan that of each of the second and third IDT electrodes 103 and 104,and the number of electrode fingers of the second IDT electrode 103 andthat of the third IDT electrode 104 are equal to each other.

If the pitch of the first IDT electrode 102 is represented by P1 and thepitch of the second and third IDT electrode 103 and 104 is representedby P2, the relationship between P1 and P2 is P1>P2.

The first IDT electrode 102 and each of the second and third IDTelectrode 103 and 104 have metalization ratios η equal to each other.The metalization ratio η represents the proportion of the width of theelectrode finger in one wavelength.

The metalization ratio η is expressed by the following equation (1).

η=L/(L+S)  [Equation 1]

where L is the width of one electrode finger and S is the spacing fromthis electrode finger to the next electrode finger.

The operation of this embodiment will now be described.

FIG. 2 shows the relationship between the number and the metalizationratio η of electrode fingers of the first IDT electrode 102 and peakfrequencies of radiation characteristics of the first IDT electrode 102.A peak frequency of a radiation characteristic is defined as a frequencyat which level of the radiation is peaked. That is, the radiationcharacteristic of the first IDT electrode 102 is measured with respectto particular values of the number and the metalization ratio η ofelectrode fingers of the first IDT electrode 102 when the pitch is fixedto obtain the peak frequency of the radiation characteristic, and thecharacteristics curves shown in FIG. 2 are formed by plotting the centerfrequencies obtained.

As described above, while the number of electrode fingers and themetalization ratio η of the first IDT electrode 102 are changed in theprocess of obtaining the relationship shown in FIG. 2, the electrodefinger pitch is fixed.

As is apparent from FIG. 2, when the number of electrode fingers of thefirst IDT electrode 102 is more increased, the peak frequency of theradiation characteristic of the first IDT electrode 102 is higher. Also,if the metalization ratio η of the first IDT electrode 102 is increased,the center frequency of the radiation characteristic of the first IDTelectrode 102 is lower. Thus, the peak frequency of the radiationcharacteristic of the first IDT electrode 102 becomes higher when thenumber of electrode fingers is more increased or when the metalizationratio η is more reduced while the electrode film thickness of the firstIDT electrode 102 is constant. With respect to the second and third IDTelectrodes 103 and 104, the same tendency as that observed with respectto the first IDT electrode 102 is recognized.

In FIG. 4, the radiation characteristic of the first IDT electrode 102is indicated by 401. As clearly shown in FIG. 4, the radiationcharacteristic of the first IDT electrode 102 is asymmetric about thecenter frequency because of the influence of a reflectioncharacteristic.

The radiation characteristic of the second IDT electrode 103 isindicated by 402 in FIG. 4. The radiation characteristic of the secondIDT electrode 103 is asymmetric about the center frequency because ofthe influence of a reflection characteristic, as is that of the firstIDT electrode 102. Since the third IDT electrode 104 has the same numberof electrode fingers as the second IDT electrode 103, the radiationcharacteristic of the third IDT electrode 104 is the same as that of thesecond IDT electrode 103. Therefore the radiation characteristic of thethird IDT electrode 104, as well as that of the second IDT electrode103, is indicated by 402 in FIG. 4.

On the other hand, the inventor of the present invention found the factthat the surface acoustic wave filter has an improved characteristic ifit is designed so that the center frequency of the radiationcharacteristic 401 of the first IDT electrode 102 and the centerfrequency of the radiation characteristic 402 of the second and thirdIDT electrodes 103 and 104 are equal to each other, i.e. the fact thatif the surface acoustic wave filter is designed in this manner, it hasan attenuation characteristic of a wider band.

As mentioned above, the number of electrode fingers of the first IDTelectrode 102 is larger than that of each of the second and third IDTelectrodes 103 and 104. Therefore, as is apparent from FIG. 2, if thepitch of the electrode fingers of the first IDT electrode 102 is equalto that of the electrode fingers of each of the second and third IDTelectrodes 103 and 104, the center frequency of the radiationcharacteristic of the first IDT electrode 102 is higher than that of theradiation characteristic of the second and third IDT electrodes 103 and104.

However, as is apparent from FIG. 2, the center frequency of theradiation characteristic of the first IDT electrode 102 and that of theradiation characteristic of the second and third IDT electrodes 103 and104 can be set approximately equal to each other by establishing therelationship P1>P2. More specifically, while there is a difference ofabout 0.9% between the center frequencies of the radiationcharacteristics when P1=P2, the difference between the centerfrequencies of the radiation characteristics can be reduced to about0.5% and, preferably, to 0.1% by establishing the relationship P1>P2.That is, the pitch of the group of electrode fingers larger in number inthe groups of electrode fingers of the IDT electrodes constituting thesurface acoustic wave filter shown in FIG. 1 may be adjusted so as to belarger than the pitch of the other groups of electrode fingers smallerin number to set the radiation characteristics of the IDT electrodes incorrespondence with each other.

The surface acoustic wave filter is thus arranged to be realized as awide-band filter having a steep attenuation characteristic.

While this embodiment has been described by assuming that the inputterminal IN is of an unbalanced type, the input terminal IN is notlimited to the unbalanced type. A balanced type of input terminal IN mayalternatively be used.

While this embodiment has been described by assuming that the outputterminal OUT is an unbalanced type, the output terminal OUT is notlimited to the unbalanced type. A balanced type of output terminal OUTmay alternatively be used.

This embodiment has been described with respect to the case where eachelectrodes in the second and third IDT electrodes 103 and 104 isconnected to the input terminal IN while the first IDT electrode 102 isconnected to the output terminal OUT. However, the arrangement mayalternatively be such that each electrodes in the second and third IDTelectrodes 103 and 104 is connected to the output terminal OUT while thefirst IDT electrode 102 is connected to the input terminal IN.

This embodiment has been described by assuming that the number offingers of the second and third IDT electrodes are equal to each other.However, if these numbers are different, the IDT electrodes may beadjusted so that their radiation characteristics coincide with eachother.

(Second Embodiment)

A second embodiment of the present invention will be described.

FIG. 3 schematically shows a surface acoustic wave filter of thisembodiment.

Referring to FIG. 3, a first filter track 307 is formed on apiezoelectric substrate 301 by first, second, and third IDT electrodes302,303, and 304, and first and second reflector electrodes 305 and 306.That is, the second IDT electrodes 303 and the third IDT electrodes 304are placed on the both sides of the first IDT electrodes 302. Thereflector electrode 305 is placed on the side of the second IDTelectrode 303 opposite from the side on which the first IDT electrode302 are placed. The reflector electrode 306 is placed on the side of thethird IDT electrode 304 opposite from the side on which the first IDTelectrode 302 are placed.

Also, a second filter track 313 is formed by fourth, fifth, and sixthIDT electrodes 308,309, and 310, and first and second reflectorelectrodes 311 and 312. That is, the fifth IDT electrode 309 and thesixth IDT electrode 310 are placed on the opposite sides of the fourthIDT electrode 308. The reflector electrode 311 is placed on the side ofthe fifth IDT electrode 309 opposite from the side on which the fourthIDT electrode 308 are placed. The reflector electrode 312 is placed onthe side of the sixth IDT electrode 310 opposite from the side on whichthe fourth IDT electrode 308 a replaced.

The upper electrode fingers of the first IDT electrode 302 is connectedto an input terminal IN. The lower electrode fingers of the second IDTelectrode 303 is connected to the upper electrode fingers of the fifthIDT electrode 309, and the lower electrode fingers of the third IDTelectrode 304 is connected to the upper electrode fingers of the sixthIDT electrode 310. The upper electrode fingers of the fourth IDTelectrode 308 is connected to an output terminal OUT1, while the lowerelectrode fingers of the fourth IDT electrode 308 is connected toanother output terminal OUT2.

The number of electrode fingers of the first IDT electrode 302 is equalto that of the fourth IDT electrode 308. The second IDT electrode 303,the third IDT electrode 304, the fifth IDT electrode 309 and the sixthIDT electrode 310 have numbers of electrode fingers equal to each other.

The number of electrode fingers of each of the first and fourth IDTelectrodes 302 and 308 is larger than that of each of the second, third,fifth, and sixth IDT electrodes 303, 304, 309, and 310. The pitch of theelectrode fingers of the first and fourth IDT electrodes 302 and 308 isrepresented by P1 and the pitch of the electrode fingers of the second,third, fifth, and sixth IDT electrodes 303, 304, 309, and 310 isrepresented by P2. The relationship between P1 and P2 is P1>P2. Themetalization ratios of all of IDT electrodes are equal to each other.The surface acoustic wave filter of this embodiment is thus constructedas a two-stage longitudinal-mode surface acoustic wave filter.

The operation of this embodiment will be described.

FIG. 8(a) shows radiation characteristics of the first and second IDTelectrodes 302 and 303. The radiation characteristic of the first IDTelectrode 302 is indicated by 1801 and the radiation characteristic ofthe second IDT electrode 303 is indicated by 1802. As is apparent fromFIG. 8, the peak frequency of the radiation characteristic 1801 and thepeak frequency of the radiation characteristic 1802 coincide with eachother. The peak frequencies fp of the radiation characteristics of thefirst and second IDT electrodes 302 and 303 can be set approximatelyequal to each other by establishing the relationship P1>P2, as in thefirst embodiment. Also, the peak frequency fp of the radiationcharacteristic of the first IDT electrode 302 can be set approximatelyequal to the peak frequency fp of the radiation characteristic of thethird IDT electrode 304 by establishing the relationship P1>P2. Further,the peak frequency fp of the radiation characteristic of the fourth IDTelectrode 308 can be set approximately equal to the peak frequency fp ofthe radiation characteristic of each of the fifth and sixth IDTelectrodes 309 and 310 by establishing the relationship P1>P2.

FIG. 8(b) shows a pass characteristic of the surface acoustic wavefilter of this embodiment indicated by 1803 a and by 1803 b. The passcharacteristic of the surface acoustic wave filter of this embodiment isindicated by 1803 a with respect to a wide gain range from 0 dB to 90dB, and a central portion of the characteristic curve 1803 a isindicated by 1803 b with respect to a narrow gain range from 0 dB to 10dB. Frequency fp is the peak frequency of the radiation characteristicof each IDT electrodes. This filter pass characteristic of the surfaceacoustic wave filter is exhibited when the peak frequency fp of theradiation characteristic of each IDT electrodes is set in correspondencewith the left end of the pass band, i.e., the lower limit frequency ofthe pass band.

FIG. 9(a) shows radiation characteristics of IDT electrodes in aconventional surface acoustic wave filter. In FIG. 9(a), the first andfourth IDT electrodes 302 and 308 have the radiation characteristicindicated by 1901, and the second, third, fifth, and sixth IDTelectrodes 303, 304, 309, and 310 have the radiation characteristicindicated by 1902. That is, in the conventional surface acoustic wavefilter, the center frequency fp1 of the first and fourth IDT electrodes302 and 308 and the center frequency fp2 of the second, third, fifth,and sixth IDT electrodes 303, 304, 309, and 310 do not coincide witheach other.

FIG. 9(b) shows a pass characteristic of this surface acoustic wavefilter indicated by 1903 a and by 1903 b. The pass characteristic of theconventional surface acoustic wave filter is indicated by 1903 a withrespect to a wide gain range from 0 dB to 90 dB, and a central portionof the characteristic curve 1903 a is indicated by 1903 b with respectto a narrow gain range from 0 dB to 10 dB.

In the pass characteristic 1803 b shown in FIG. 8(b) an improvement isrecognized in comparison with the pass characteristic 1903 b shown inFIG. 9(b), such that a cut of the pass band at the left end is reducedto widen the pass band. This means a reduction in loss at the band end.

Thus, the center frequencies of the radiation characteristics of the IDTelectrodes are set approximately equal to each other to realize asurface acoustic wave filter having a characteristic of a wider band.

As described above, according to this embodiment, a wide-band surfaceacoustic wave filter having a steep attenuation characteristic can berealized.

While the output terminals form a balanced output in this embodiment,the same effect of the present invention can also be achieved even ifthe upper or lower one of the fourth IDT electrode may be grounded toform an unbalanced output.

(Third Embodiment)

A third embodiment of the present invention will be described.

FIG. 6 schematically shows a surface acoustic wave filter of thisembodiment. Referring to FIG. 6, the surface acoustic wave filter has apiezoelectric substrate 601, first, second, and third IDT electrodes602, 603, and 604, and first and second reflector electrodes 605 and606, the IDT electrodes and the reflector electrodes being formed on thesubstrate.

The second IDT electrode 603 and the third IDT electrode 604 are placedon the both sides of the first IDT electrode 602. The reflectorelectrode 605 is placed on the side of the second IDT electrode 603opposite from the side on which the first IDT electrode 602 are placed.The reflector electrode 606 is placed on the side of the third IDTelectrode 604 opposite from the side on which the first IDT electrode602 are placed.

The upper electrode fingers of each of the second and third IDTelectrodes 603 and 604 is connected to an input terminal IN, while thelower electrode fingers of each of the second and third IDT electrodes603 and 604 is grounded. The lower electrode of the first IDT electrode602 is connected to an output terminal OUT, while the upper one of thefirst IDT electrode 602 is grounded.

The number of electrode fingers of the first IDT electrode 602 is largerthan that of each of the second and third IDT electrodes 603 and 604,and the number of electrode fingers of the second IDT electrode 603 andthat of the third IDT electrode 604 are equal to each other.

The pitch of the electrode fingers of the first IDT electrode 602 isrepresented by P1 and the pitch of the electrode fingers of the secondand third IDT electrodes 603 and 604 is represented by P2. Therelationship between P1 and P2 is P1>P2.

In the surface acoustic wave filter of this embodiment, the first IDTelectrode 602 and each of the second and third IDT electrodes 603 and604 have metalization ratios different from each other.

The operation of this embodiment will be described.

The number of electrode fingers of the first IDT electrode 602 is largerthan that of each of the second and third IDT electrodes 603 and 604.Therefore, as is apparent from FIG. 2, if the metalization ratio of thefirst IDT electrode 602 is equal to that of the second and third IDTelectrodes 603 and 604, the center frequency of the radiationcharacteristic of the first IDT electrode 602 is higher than that of theradiation characteristic of the second and third IDT electrodes 603 and604.

However, as is apparent from FIG. 2, the peak frequency of the radiationcharacteristic of the first IDT electrode 602 and that of the radiationcharacteristic of the second and third IDT electrodes 603 and 604 can beset approximately equal to each other by establishing the relationshipP1>P2 and by making the metalization ratio of the first IDT electrode602 and the metalization ratio of the second and third IDT electrodes603 and 604 different values.

In this embodiment, as described above, the center frequency of theradiation characteristic of the first IDT electrode 602 and the centerfrequency of the radiation characteristic of the second and third IDTelectrodes 603 and 604 can be set approximately equal to each other byadjusting the pitch and the metalization ratio of each IDT electrodes.Since adjustment of the metalization of each IDT electrode, i.e.,adjustment of the intensity of surface acoustic wave excitation and thereflection on each IDT electrodes, is also performed, the design freedomis improved in comparison with the first embodiment.

While in this embodiment the pitches are adjusted so that P1>P2, beingnot exclusively used adjustment of the metalization ratio mayalternatively be performed while the pitches are set in the relationshipP1=P2.

The surface acoustic wave filter is thus arranged to be realized as awide-band filter having a steep attenuation characteristic.

While this embodiment has been described by assuming that the inputterminal IN is of an unbalanced type, the input terminal IN is notlimited to the unbalanced type. A balanced type of input terminal IN mayalternatively be used.

While this embodiment has been described by assuming that the outputterminal OUT is an unbalanced type, the output terminal OUT is notlimited to the unbalanced type. A balanced type of output terminal OUTmay alternatively be used.

This embodiment has been described with respect to the case where thesecond and third IDT electrodes 603 and 604 is connected to the inputterminal IN while the first IDT electrode 602 is connected to the outputterminal OUT. However, the arrangement may alternatively be such thatthe second and third IDT electrodes 603 and 604 is connected to theoutput terminal OUT while the first IDT electrode 602 is connected tothe input terminal IN.

(Fourth Embodiment)

A fourth embodiment of the present invention will be described.

FIG. 7 schematically shows a surface acoustic wave filter of thisembodiment.

Referring to FIG. 7, a first filter track 707 is formed on apiezoelectric substrate 701 by first, second, and third IDT electrodes702, 703, and 704 and first and second reflector electrodes 705 and 706.That is, the second IDT electrode 703 and the third IDT electrode 704are placed on the both sides of the first IDT electrode 702. Thereflector electrode 705 is placed on the side of the second IDTelectrode 703 opposite from the side on which the first IDT electrode702 are placed. The reflector electrode 706 is placed on the side of thethird IDT electrode 704 opposite from the side on which the first IDTelectrode 702 are placed.

Also, a second filter track 713 is formed by fourth, fifth, and sixthIDT electrodes 708, 709, and 710, and first and second reflectorelectrodes 711 and 712. That is, the fifth IDT electrode 709 and thesixth IDT electrode 710 are placed on the both sides of the fourth ofIDT electrode 708. The reflector electrode 711 is placed on the side ofthe fifth IDT electrode 709 opposite from the side on which the fourthIDT electrode 708 are placed. The reflector electrode 712 is placed onthe side of the sixth IDT electrode 710 opposite from the side on whichthe fourth IDT electrode 708 are placed.

The upper electrode fingers of the first IDT electrodes 702 is connectedto an input terminal IN. The lower electrode fingers of the IDTelectrode 703 is connected to the upper electrode fingers of the fifthIDT electrode 709, and the lower electrode fingers of the third IDTelectrode 704 is connected to the upper electrode fingers of the sixthIDT electrode 710. The upper electrode fingers of the fourth IDTelectrode 708 is connected to an output terminal OUT1, while the lowerelectrode fingers of the fourth IDT electrode 708 is connected toanother output terminal OUT2.

The number of electrode fingers of the first IDT electrode 702 is equalto that of the fourth IDT electrode 708. The second IDT electrode 703,the third IDT electrode 704, the fifth IDT electrode 709 and the sixthIDT electrode 710 have numbers of electrode fingers equal to each other.

The number of electrode fingers of each of the first and fourth IDTelectrodes 702 and 708 is larger than that of each of the second, third,fifth, and sixth IDT electrodes 703, 704, 709, and 710. The pitch of theelectrode fingers of the first IDT electrode 702 is represented by P11and the pitch of the electrode fingers of the second and third IDTelectrodes 703 and 704 is indicated by P12. The relationship between P11and P12 is P11>P12. Also, the pitch of the electrode fingers of thefourth IDT electrode 708 is represented by P21 and the pitch of theelectrode fingers of the fifth and sixth IDT electrodes 709 and 710 isindicated by P22. The relationship between P21 and P22 is P21>P22.

The metalization ratio η11 of the first IDT electrode 702 in the firstfilter track 707 is expressed by the following equation (2):

η11 =L 11/(L 11 +S 11)  [Equation 2]

where L11 is the width of the electrode fingers of the first IDTelectrode 702, and S11 is the spacing from one electrode finger to thenext electrode finger in the first IDT electrodes 702.

The metalization ratio η12 of the second and third IDT electrodes 703and 704 is expressed by the following equation (3):

η12 =L 12/(L 12 +S 12)  [Equation 3]

where L12 is the width of the electrode fingers of the second and thirdIDT electrodes 703 and 704, and S12 is the spacing from one electrodefinger to the next electrode finger in the second and third IDTelectrodes 703 and 704.

The metalization ratio η21 of the fourth IDT electrode 708 in the secondfilter track 708 is expressed by the following equation (4):

η21 =L 21/(L 21 +S 21)  [Equation 4]

where L21 is the width of the electrode fingers of the fourth IDTelectrode 708, and S21 is the spacing from one electrode finger to thenext electrode finger in the fourth IDT electrode 708.

The metalization ratio η22 of the fifth and sixth IDT electrodes 709 and710 is expressed by the following equation (5):

η22 =L 22/(L 22 +S 22)  [Equation 5]

where L22 is the width of the electrode fingers of the fifth and sixthIDT electrodes 709 and 710, and S22 is the spacing from one electrodefinger to the next electrode finger in the fifth and sixth IDTelectrodes 709 and 710.

In this embodiment, η11 and η12 shown above are different from eachother, and η21 and η22 shown above are also different from each other.

The surface acoustic wave filter of this embodiment is thus constructedas a two-stage longitudinal-mode surface acoustic wave filter.

The operation of this embodiment will be described.

The center frequency fp of the radiation characteristic of the first IDTelectrode 702 and the center frequency fp of the radiationcharacteristic of the second and third IDT electrodes 703 and 704 areset approximately equal to each other by establishing the relationshipP11>P12 and by adjusting η11 and η12 to different values in accordancewith the same method as that described above in detail in thedescription of the embodiments.

Also, the peak frequency fp of the radiation characteristic of thefourth IDT electrode 708 and the peak frequency fp of the radiationcharacteristic of the fifth and sixth IDT electrodes 709 and 710 are setapproximately equal to each other by establishing the relationshipP21>P22 and by adjusting η21 and η22 to different values.

Consequently, the surface acoustic wave filter of this embodiment can berealized as a wide-band surface acoustic wave filter having a steepattenuation characteristic, as are those in the above-describedembodiments.

A wide band surface acoustic wave filter having a steep attenuationcharacteristic can be realized by being arranged as described above.

This embodiment has been described with respect to the case where η11and η12 are adjusted to different values while the relationship P11>P12is established, and where η21 and η22 are adjusted to different valueswhile the relationship P21>P22 is established. However, this adjustmentmethod is not exclusively used. Adjustment using only the relationshipP11>P21 and the relationship P21>P22 may alternatively be performed.Also, adjustment by changing only the metalization ratio whileestablishing P11<P12 and P21=P22 may be performed.

While the output terminals form a balanced output in this embodiment,the same effect of the present invention can also be achieved even ifthe upper or lower one of the fourth IDT electrode 708 may be groundedto form an unbalanced output.

(Fifth Embodiment)

A fifth embodiment of the present invention will be described.

FIG. 10 schematically shows a surface acoustic wave filter of thisembodiment.

Referring to FIG. 10, the surface acoustic wave filter has apiezoelectric substrate 1001, first, second, and third IDT electrodes1002, 1003, and 1004, and first and second reflector electrodes 1005 and1006, the IDT electrodes and the reflector electrodes being formed onthe substrate.

The second IDT electrode 1003 and the third IDT electrodes 1004 areplaced on the opposite sides of the first IDT electrode 1002. Thereflector electrode 1005 is placed on the side of the second IDTelectrode 1003 opposite from the side on which the first IDT electrode1002 are placed. The reflector electrode 1006 is placed on the side ofthe third IDT electrodes 1004 opposite from the side on which the firstIDT electrode 1002 are placed. Thus, the first, second, and third IDTelectrodes 1002, 1003, and 1004 and the first and second reflectorelectrodes 1005 and 1006 are arranged along the direction of propagationof a surface acoustic wave.

The upper electrode fingers of each of the second and third IDTelectrodes 1003 and 1004 is connected to an input terminal IN, while thelower electrode fingers of each of the second and third IDT electrodes1003 and 1004 is grounded. The lower electrode fingers of the first IDTelectrode 1002 is connected to an output terminal OUT, while the upperelectrode fingers of the first IDT electrode 1002 is grounded.

The number of electrode fingers of the first IDT electrode 1002 islarger than that of each of the second and third IDT electrodes 1003 and1004, and the number of electrode fingers of the second IDT electrode1003 and that of the third IDT electrode 1004 are equal to each other.

If the pitch in a region indicated by 1 a in the first IDT electrode1002 is P1, P1 is ½ wavelength. Also, if the pitch in a region indicatedby 1 b is P1′, P1′ is smaller than ½ wavelength. The number of electrodefingers in the region 1 a having the pitch P is larger than the numberof electrode fingers in the region 1 b having the pitch P1′. In thefirst IDT electrode 1002, therefore, the region indicated by 1 a is amain excitation region.

Also, if the pitch in a region indicated by 2 a in the second IDTelectrode 1003 is P2, P2 is ½ wavelength. Also, if the pitch in a regionindicated by 2 b is P2′, P2′ is smaller than ½ wavelength. The number ofelectrode fingers in the region 2 a having the pitch P2 is larger thanthe number of electrode fingers in the region 2 b having the pitch P2′.In the second IDT electrode 1003, therefore, the region indicated by 2 ais a main excitation region.

The pitch in a region indicated by 3 a in the third IDT electrode 1004is P2, and P2 is ½ wavelength. The pitch in a region indicated by 3 b isP2′, and P2′ is smaller than ½ wavelength. The number of electrodefingers in the region 3 a having the pitch P2 is larger than the numberof electrode fingers in the region 3 b having the pitch P2′. In thethird IDT electrode 1004, therefore, the region indicated by 3 a is amain excitation region.

Thus, in each of the first IDT electrode 1002, the second IDT electrode1003 and the third IDT electrode 1004, different electrode fingerpitches are set between the electrode fingers of the same IDT electrode.

If the relationship between the pitch P1 and P2 satisfies P1>P2, thesame effect as that of the first embodiment can be achieved.

In the case where the relationship between the pitch P1 and P2 satisfiesP1>P2, the relationship between the pitch P1′ and P2′ may satisfyP1′>P2′ or P1′=P2′. The discontinuity between the adjacent electrodefingers when P1′>P2′ is satisfied can be smaller than that when P1′=P2′is satisfied. The insertion loss can be relatively reduced by satisfyingP1′>P2′.

While this embodiment has been described with respect to the case whereP1>P2 is satisfied, the P1 and P2 may be adjusted so that the respectivepeak frequencies of radiation characteristics of the regions 1 a, 2 a,and 3 a shown in FIG. 10 are set approximately equal to each other. Itis desirable that the pitch P1, P1′, P2, and P2′ be adjusted so that theradiation characteristics of the first, second, and third IDT electrodes1002, 1003, and 1004 coincide with each other. In such a case, equalitybetween the pitches p2 and P2′ of the second and third IDT electrodes1003 and 1004 is not necessarily required.

To an arrangement in which the electrode fingers of one IDT electrodehave different pitches as described above, each of the above-describedembodiments may be applied on the basis of a setting of the pitches ofthe main excitation electrode fingers in the main excitation regions.

A wide-band surface acoustic wave filter having a steep attenuationcharacteristic can be realized by being arranged as described above.

While this embodiment has been described by assuming that the inputterminal IN is of an unbalanced type, the input terminal IN is notlimited to the unbalanced type. A balanced type of input terminal IN mayalternatively be used.

While this embodiment has been described by assuming that the outputterminal OUT is an unbalanced type, the output terminal OUT is notlimited to the unbalanced type. A balanced type of output terminal OUTmay alternatively be used.

This embodiment has been described with respect to the case where thesecond and third IDT electrodes 1003 and 1004 is connected to the inputterminal IN while the first IDT electrode 1002 is connected to theoutput terminal OUT. However, the arrangement may alternatively be suchthe second and third IDT electrodes 1003 and 1004 is connected to theoutput terminal OUT while the first IDT electrode 1002 is connected tothe input terminal IN.

(Sixth Embodiment)

A sixth embodiment of the present invention will be described.

FIG. 11 schematically shows a surface acoustic wave filter of thisembodiment.

Referring to FIG. 11, the surface acoustic wave filter has apiezoelectric substrate 101, first, second, third, fourth, and fifth IDTelectrodes 1102, 1103, 1104, 1105, and 1106, and first and secondreflector electrodes 1107 and 1108, the IDT electrodes and the reflectorelectrodes being formed on the substrate.

The second IDT electrode 1103 and the third IDT electrode 1104 areplaced on the both sides of the first IDT electrode 1102. The fifth IDTelectrode 1106 are placed on the side of the third IDT electrode 1104opposite from the side on which the first IDT electrode 1102 are placed.The fourth IDT electrode 1105 are placed on the side of the second IDTelectrode 1103 opposite from the side on which the first IDT electrode1102 are placed. The first reflector electrode 1107 is placed outsidethe fourth IDT electrode 1105, and the second reflector electrode 1108is placed outside the fifth IDT electrode 1106.

Thus, the first, second, third, fourth, and fifth IDT electrodes 1102,1103, 1104, 1105, and 1106 and the first and second reflector electrodes1107 and 1108 are arranged along the direction of propagation of asurface acoustic wave.

The lower electrode fingers of each of the second and third IDTelectrodes 1103 and 1104 is connected to an output terminal OUT, whilethe upper electrode fingers of each of the second and third IDTelectrodes 1103 and 1104 is grounded. The upper electrode fingers ofeach of the first, fourth, and fifth IDT electrodes 1102, 1105, and 1106is connected to an input terminal IN, while the lower electrode fingersof each of the first, fourth, and fifth IDT electrodes 1102, 1105, and1106 is grounded.

The number of electrode fingers of the first IDT electrode 1102 islarger than that of each of the second and third IDT electrodes 1103 and1104, and the number of electrode fingers of the second IDT electrode1103 and that of the third IDT electrode 1104 are equal to each other.Also, the number of electrode fingers of each of the fourth and fifthIDT electrodes 1105 and 1106 is smaller than that of each of the secondand third IDT electrodes 1103 and 1104, and the number of electrodefingers of the fourth IDT electrode 1105 and that of the fifth IDTelectrode 1106 are equal to each other.

If the pitch of the electrode fingers of the first of IDT electrode 1102is P1; the pitch of the electrode fingers of the second and third IDTelectrodes 1103 and 1104 is P2; and the pitch of the electrode fingersof the fourth and fifth IDT electrodes 1105 and 1106 is P3, the pitchesP1 to P3 are in the relationship P1>P2>P3. That is, the pitch of onegroup of electrode fingers larger in number in the groups of electrodefingers of the IDT electrodes constituting the surface acoustic wavefilter of this embodiment is larger than the pitch of another group ofelectrode fingers smaller in number.

The first, second, third, fourth, and fifth IDT electrodes 1102, 1103,1104, 1105, and 1106 have metalization ratios η equal to each other. Themetalization ratio η represents the proportion of the width of theelectrode finger in one wavelength.

The metalization ratio η is expressed by the equation (1) shown above inthe description of the first embodiment.

The operation of this embodiment will be described.

If the number of electrode fingers of the first IDT electrode 1102 isincreased, the center frequency of the radiation characteristic of thefirst IDT electrode 1102 is higher, as described above in thedescription of the first embodiment. Also, if the metalization ratio ηof the first IDT electrode 1102 is increased, the center frequency ofthe radiation characteristic of the first IDT electrode 1102 is lower.Thus, the center frequency of the radiation characteristic of the firstIDT electrode 1102 becomes higher if the number of electrode fingers isincreased or if the metalization ratio η is reduced while the electrodefilm thickness of the first IDT electrode 1102 is constant. With respectto the second, third, fourth, and fifth IDT electrodes 1103, 1104,1105,and 1106, the same tendency as that observed with respect to the firstIDT electrode 1102 is recognized.

Since the pitch P1 of the electrode fingers of the first IDT electrode1102, the pitch P2 of the electrode fingers of each of the second andthird IDT electrodes 1103 and 1104, and the pitch P3 of the electrodefingers of each of the fourth and fifth IDT electrodes 1105 and 1106 arein the relationship P1>P2>P3, the center frequencies of radiationcharacteristics of the first, second, third, fourth, and fifth IDTelectrodes 1102, 1103, 1104, 1105, and 1106 can be set approximatelyequal to each other.

That is, if the center frequencies of the radiation characteristics ofthe first, second, third, fourth, and fifth IDT electrodes 1102, 1103,1104, 1105, and 1106 are equal to each other, the surface acoustic wavefilter of this embodiment has an attenuation characteristic of a widerband.

The surface acoustic wave filter is thus arranged to be realized as awide-band filter having a steep attenuation characteristic.

While this embodiment has been described by assuming that the inputterminal IN is of an unbalanced type, the input terminal IN is notlimited to the unbalanced type. A balanced type of input terminal IN mayalternatively be used.

While this embodiment has been described by assuming that the outputterminal OUT is an unbalanced type, the output terminal OUT is notlimited to the unbalanced type. A balanced type of output terminal OUTmay alternatively be used.

The relationship between the numbers of electrode fingers of the firstto fifth IDT electrodes is not limited to that described above. Therelationship is optimized according to filter characteristics.

This embodiment has been described with respect to the case where thesecond and third IDT electrodes 1103 and 1104 is connected to the outputterminal OUT while the first, fourth, and fifth IDT electrodes 1102,1105, and 1106 is connected to the input terminal IN. However, thearrangement may alternatively be such that the second and third IDTelectrodes 1103 and 1104 is connected to the input terminal IN while thefirst, fourth, and fifth IDT electrodes 1102, 1105, and 1106 isconnected to the output terminal OUT.

This embodiment has been described with respect to the case where thepitch of the electrode fingers of each IDT electrodes is adjusted andthe case where the electrode finger pitch and the metalization ratio ofeach IDT electrodes are adjusted. However, the present invention is notlimited to the described adjustment methods. The center frequencies ofthe radiation characteristics of the IDT electrodes can be setapproximately equal to each other in a different manner as describedbelow.

It is known that the peak frequency of the radiation characteristic ofeach IDT electrodes becomes lower if the film thickness of the IDTelectrodes is increased. Therefore it is possible to adjust the peakfrequencies of the radiation characteristics of the IDT electrodes tothe desired frequency by setting the film thickness of the first IDTelectrode and the film thickness of the second and third IDT electrodesto different values.

It is also known that the peak frequency of the radiation characteristicof each IDT electrodes is changed if the material of the IDT electrodesis changed. Therefore it is possible to adjust the peak frequencies ofthe radiation characteristics of the IDT electrodes to the desiredfrequency by using different materials for the first IDT electrode andthe second and third IDT electrodes.

It is also known that the peak frequency of the radiation characteristicof each IDT electrodes becomes lower if the metalization ratio of theIDT electrodes is increased. Therefore it is possible to adjust the peakfrequencies of the radiation characteristics of the IDT electrodes tothe desired frequency by setting the metalization ratio of the first IDTelectrode and the metalization of the second and third IDT electrodes todifferent values.

It is also possible to adjust the center frequencies of the radiationcharacteristics of the IDT electrode to the desired frequency by freelycombining the above-described methods.

Each of the first to fourth embodiments has been described by assumingthat the second and third IDT electrodes have numbers of electrodefingers equal to each other. However, the present invention is notlimited to this arrangement. Even in a case where the second and thirdIDT electrodes have different numbers of electrode fingers, the sameeffect as that of the first to fourth embodiments can also be achievedif the electrode finger pitches are adjusted so that the peakfrequencies of the radiation characteristics are equal to each other.

A communication device using the surface acoustic wave filter of thepresent invention in a portion of a transmitting circuit or receivingcircuit also belongs to the present invention. Examples of such acommunication device are a portable telephone terminal, a base stationfor portable telephone terminals, a motor vehicle telephone terminal, aterminal in a Personal Handy phone System, and a radar device.

A method of manufacturing a surface acoustic wave filter having apiezoelectric substrate, IDT electrodes for input, arranged on thepiezoelectric substrate, and IDT electrodes for output, arranged on thepiezoelectric substrate, in which the peak frequency of the radiationcharacteristic of the input IDT electrodes and the peak frequency of theradiation characteristic of the output IDT electrodes are setsubstantially equal to each other also belongs to the present invention.

A method of manufacturing a surface acoustic wave filter having apiezoelectric substrate, IDT electrodes for input, arranged on thepiezoelectric substrate, and IDT electrodes for output, arranged on thepiezoelectric substrate, in which the pitch of the electrode fingers ofthe input IDT electrodes and the pitch of the electrode fingers of theoutput IDT electrodes are set different from each other also belongs tothe present invention.

The output IDT electrodes of the present invention are not limited tothe first IDT electrode of the surface acoustic wave filter in theembodiment described above with reference to FIG. 1 or 6, or to thefourth, fifth, and sixth IDT electrodes of the surface acoustic wavefilter described above with reference to FIG. 3 or 7. Also, the inputIDT electrodes of the present invention are not limited to the secondand third IDT electrodes of the surface acoustic wave filter in theembodiment described above with reference to FIG. 1 or 6, or to thefirst, second, and third IDT electrodes of the surface acoustic wavefilter described above with reference to FIG. 3 or 7.

The embodiments of the present invention have been described withrespect to the arrangement in which three IDT electrodes are formed asinput and output IDT electrodes, and the arrangement in which five IDTelectrodes are formed as input and output IDT electrodes. However, thepresent invention is not limited to these arrangements. Two IDTelectrodes, four IDT electrodes or seven or more IDT electrodes may beformed as input and output IDT electrodes.

This embodiment has been described by assuming that the number ofelectrode fingers of the second IDT electrodes and that of the third IDTelectrode are equal to each other, and that the number of electrodefingers of the fourth IDT electrode and that of the fifth IDT electrodesare equal to each other. However, if these numbers are different, theIDT electrodes may be adjusted so that their radiation characteristicscoincide with each other.

As is apparent from the foregoing, the present invention makes itpossible to provide a surface acoustic wave filter of a wider bandhaving a steep out-of-band attenuation characteristic, a method ofmanufacturing the surface acoustic wave filter, and a communicationdevice using the surface acoustic wave filter.

What is claimed is:
 1. A surface acoustic wave filter comprising: apiezoelectric substrate; at least an input IDT electrode arranged onsaid piezoelectric substrate; and at least an output IDT electrodearranged on said piezoelectric substrate; wherein said surface acousticwave filter is a longitudinally coupled mode surface acoustic filter andincludes said input IDT electrode and said output IDT electrode disposedwithin a single propagation path in which a surface acoustic wavepropagates, a pitch of electrode fingers of said input IDT electrode anda pitch of electrode fingers of said output IDT electrode are differentfrom each other; one of said input IDT electrode and said output IDTelectrode comprises a first IDT electrode including a pair of electrodefingers opposed to each other; the other of said input IDT electrode andsaid output IDT electrode comprises a second IDT electrode including apair of electrode fingers opposed to each other, and a third IDTelectrode including a pair of electrode fingers opposed to each other,said second IDT electrode being placed on one side of said first IDTelectrode, said third IDT electrode being placed on the other side ofsaid first IDT electrode; said first, second, and third IDT electrodesare arranged alone a direction in which a surface acoustic wavepropagates; the metalization ratio of said first IDT electrode and themetalization ratio of each of said second and third IDT electrodes aresubstantially equal to each other; and the number of electrode fingersof said first IDT electrode is larger than the number of electrodefingers of each of said second and third IDT electrodes.
 2. The surfaceacoustic wave filter according to claim 1, wherein the pitch ofelectrode fingers of the IDT electrode larger in number of electrodefingers in said input and output IDT electrode is larger than the pitchof electrode fingers smaller in number of electrode fingers of the IDTelectrode.
 3. A surface acoustic wave filter comprising: a piezoelectricsubstrate; at least an input IDT electrode arranged on saidpiezoelectric substrate; and at least an output IDT electrode arrangedon said piezoelectric substrate, wherein said surface acoustic wavefilter is a longitudinally coupled mode surface acoustic wave filter andincludes said input IDT electrode and said output IDT electrode disposedwithin a single propagation path in which a surface acoustic wavepropagates, said input IDT electrode has a plurality of electrodefingers pitches, of the electrode fingers pitches of said input IDTelectrode, an electrode fingers pitch having the most pairs of electrodefingers is defined as a main input pitch, said output IDT electrode hasa plurality of electrode fingers pitches, of the electrode fingerspitches of said outout IDT electrode, an electrode fingers pitch havingthe most pairs of electrode fingers is defined as a main outout pitch,and a metalization ratio of a part corresponding to the input IDTelectrode having the main input pitch and a metalization ratio of a partcorresponding to the output IDT electrode having the main output pitchare different from each other.
 4. A communication device comprising: atransmitting circuit which outputs a transmitted wave; and a receivingcircuit to which a wave to be received is input, wherein the surfaceacoustic wave filter according to claim 3 is used in said transmittingcircuit and/or in said receiving circuit.
 5. A surface acoustic wavefilter comprising: a piezoelectric substrate; at least an input IDTelectrode arranged on said piezoelectric substrate; and at least anoutput IDT electrode arranged on said piezoelectric substrate, whereinsaid surface acoustic wave filter is a longitudinally coupled modesurface acoustic filter and includes said input IDT electrode and saidoutput IDT electrode disposed within a single propagation path in whicha surface acoustic wave propagates, said input IDT electrode has aplurality of electrode fingers pitches; of the electrode fingers pitchesof said input IDT electrode, an electrode fingers pitch having the mostpairs of electrode fingers is defined as a main input pitch, said outputIDT electrode has a plurality of electrode fingers pitches, of theelectrode fingers pitches of said output IDT electrode, an electrodefingers pitch having the most pairs of electrode fingers is defined as amain output pitch, the main input pitch of electrode fingers of saidinput IDT electrode and the main output pitch of electrode fingers ofsaid output IDT electrode are different from each other, and the mainpitch of electrode fingers of the IDT electrode larger in number ofelectrode fingers in said input and output IDT electrode is larger thanthe main pitch of electrode fingers smaller in number of electrodefingers.
 6. A surface acoustic wave filter of claim 5 wherein said inputIDT electrode has electrode fingers snaced to define first and secondelectrode fingers pitches; the first electrode fingers pitch is the maininput pitch of said input IDT electrode, said output IDT electrode haselectrode fingers spaced to define third and fourth pitches, and thethird electrode fingers pitch is the main output pitch of said outputIDT electrode.
 7. The surface acoustic wave filter according to claim 6,wherein the metalization ratio of an IDT electrode larger in number ofelectrode fingers in said input and output IDT electrodes is lower thanthe metalization ratio of an IDT electrode smaller in number ofelectrode fingers.
 8. The surface acoustic wave filter of claim 6,wherein the first pitch is different from the second pitch, and thethird pitch is different from the fourth pitch.
 9. The surface acousticwave filter of claim 8, wherein the second pitch is different from thefourth pitch.
 10. A communication device comprising: a transmittingcircuit which outputs a transmitted wave; and a receiving circuit towhich a wave to be received is input, wherein a surface acoustic wavefilter according to claim 5 is used in said transmitting circuit and/orin said receiving circuit.
 11. The surface acoustic wave filteraccording to any one of claims 2, 3, 6 and 7, wherein a peak frequencyof a radiation characteristic of said input IDT electrode issubstantially equal to a peak frequency of a radiation characteristic ofsaid output IDT electrode.
 12. The surface acoustic wave filteraccording to claim 11, wherein one of said input IDT electrode and saidoutput IDT electrode comprises a first IDT electrode including a pair ofelectrode fingers opposed to each other; the other of said input IDTelectrode and said output IDT electrode comprises a second IDT electrodeincluding a pair of electrode fingers opposed to each other, and a thirdIDT electrode including a pair of electrode fingers opposed to eachother, said second IDT electrode being placed on one side of said firstIDT electrode, said third IDT electrode being placed on the other sideof said first IDT electrode; said first, second, and third IDTelectrodes are arranged along a direction in which a surface acousticwave propagates; and the peak frequency of the radiation characteristicof said first IDT electrode is substantially equal to the peak frequencyof the radiation characteristic of each of the second and third IDTelectrodes.
 13. The surface acoustic wave filter according to claim 12,wherein a plurality of filter tracks each having first, second, andthird IDT electrodes, and first and second reflector electrodes areformed on said piezoelectric substrate, and said plurality of filtertracks function as one filter in cooperation with each other.
 14. Thesurface acoustic wave filter according to claim 13, wherein at least oneof said plurality of filter tracks is different in configuration fromthe others.
 15. The surface acoustic wave filter according to claim 12,further comprising a first reflector electrode placed on the oppositeside of said second IDT electrode on said piezoelectric substrateopposite from the side on which said first IDT electrode are placed; anda second reflector electrode placed on the side of said third IDTelectrode on said piezoelectric substrate opposite from the side onwhich said first IDT electrode are placed, wherein said first, second,and third IDT electrodes and said first and second reflector electrodesare arranged along a direction in which a surface acoustic wavepropagates.
 16. The surface acoustic wave filter according to claim 11,wherein one of said input IDT electrode and said output IDT electrodecomprises first, fourth, and fifth IDT electrodes each including a pairof electrode fingers opposed to each other; the other of said input IDTelectrode and said output IDT electrodes comprises a second and thirdIDT electrodes each including a pair of electrode fingers opposed toeach other; said second and third IDT electrodes are placed on oppositesides of said first IDT electrode; said fourth IDT electrode are placedon the side of said second IDT electrodes opposite from the side onwhich said first IDT electrode are placed; said fifth IDT electrode areplaced on the side of said third IDT electrode opposite from the side onwhich said first IDT electrode are placed; said first, second, third,fourth and fifth IDT electrodes are arranged along a direction in whicha surface acoustic wave propagates; and the peak frequencies of theradiation characteristics of at least more than one of the group of saidfirst IDT electrode, and the group of said fourth and fifth IDTelectrodes, and the group of said second and third IDT electrodes aresubstantially equal to each other.
 17. A surface acoustic wave filtercomprising: a piezoelectric substrate: at least an input IDT electrodearranged on said piezoelectric substrate; and at least an output IDTelectrode arranged on said piezoelectric substrate; wherein said surfaceacoustic wave filter is a longitudinally coupled mode surface acousticfilter and includes said input IDT electrode and said output IDTelectrode disposed within a single propagation path in which a surfaceacoustic wave propagetes, a pitch of electrode fingers of said input IDTelectrode and a pitch of electrode fingers of said output IDT electrodeare different from each other; and a film thickness of said output IDTelectrode and a film thickness input IDT electrodes are different fromeach other.
 18. A surface acoustic wave filter comprising: apiezoelectric substrate; at least an input IDT electrode arranged onsaid piezoelectric substrate; and at least an output IDT electrodearranged on said piezoelectric substrate; wherein said surface acousticwave filter is a longitudinally coupled mode surface acoustic filter andincludes said input IDT electrode and said output IDT electrode disposedwithin a single propagation path in which a surface acoustic wavepropagates. a pitch of electrode fingers of said input IDT electrode anda pitch of electrode fingers of said output IDT electrode are differentfrom each other; and a material of said output IDT electrode and amaterial of said input IDT electrodes are different from each other. 19.A surface acoustic wave filter comprising; a piezoelectric substrate; atleast an input IDT electrode arranged on said piezoelectric substrate;and at least an output IDT electrode arranged on said piezoelectricsubstrate; wherein said surface acoustic wave filter is a longitudinallycoupled mode surface acoustic filter and includes said input IDTelectrode and said output IDT electrode disposed within a singlepropagation path in which a surface acoustic wave propagates, a pitch ofelectrode fingers of said input IDT electrode and a pitch of electrodefingers of said output IDT electrode are different from each other; oneof said input IDT electrode and said output IDT electrode comprises afirst IDT electrode including a pair of electrode fingers opposed toeach other; the other of said input IDT electrode and said output IDTelectrode comprises a second IDT electrode including a pair of electrodefingers opposed to each other, and a third IDT electrode including apair of electrode fingers opposed to each other, said second IDTelectrode being placed on one side of said first IDT electrode, saidthird IDT electrode being placed on the other side of said first IDTelectrode; said first, second, and third IDT electrodes are arrangedalone a direction in which a surface acoustic wave propagates; and themetalization ratio of said first IDT electrode, the metalization ratioof said second IDT electrode and the metalization ratio of said thirdIDT electrode are different from each other.
 20. A surface acoustic wavefilter comprising: a piezoelectric substrate; at least an input IDTelectrode arranged on said piezoelectric substrate; and at least anoutput IDT electrode arranged on said piezoelectric substrate; whereinsaid surface acoustic wave filter is a longitudinally coupled modesurface acoustic filter and includes said input IDT electrode and saidoutput IDT electrode disposed within a single propagation path in whicha surface acoustic wave propagates; a pitch of electrode fingers of saidinput IDT electrode and a pitch of electrode fingers of said output IDTelectrode are different from each other; one of said input IDT electrodeand said output IDT electrode comprises a first IDT electrode includinga pair of electrode fingers opposed to each other; the other of saidinput IDT electrode and said output IDT electrode comprises a second IDTelectrode including a pair of electrode fingers opposed to each other,and a third IDT electrode including a pair of electrode fingers opposedto each other, said second IDT electrode being placed on one side ofsaid first IDT electrode, said third IDT electrode being placed on theother side of said first IDT electrode; said first, second, and thirdIDT electrodes are arranged along a direction in which a surfaceacoustic wave propagates; a plurality of filter tracks each havingfirst, second, and third IDT electrodes, and first and second reflectorelectrodes are formed on said piezoelectric substrate, and saidplurality of filter tracks function as one filter in cooperation witheach other; and each of said plurality of filter tracks is substantiallyidentical in configuration to the others.
 21. A surface acoustic wavefilter comprising: a piezoelectric substrate; an input IDT electrodecomprising first, fourth, and fifth IDT electrodes each including a pairof electrode fingers opposed to each other; an output IDT electrodecomprising second and third IDT electrodes each including a pair ofelectrode fingers opposed to each other; the second IDT electrodedisposed between said first and fourth IDT electrodes and the third IDTelectrode disposed between said first and fifth IDT electrodes, and eachof the IDT electrodes arranged on the piezoelectric substrate; theelectrode fingers of the first IDT electrode having a first pitch; theelectrode fingers of the second and third IDT electrodes having a secondpitch; the electrode fingers of the fourth and fifth IDT electrodeshaving a third pitch; wherein the value of the pitches of the first,second, and third pitches have one of the following relationship: (a)are each different from each other; or (b) the first pitch is greaterthan the second pitch and the second pitch is greater than the thirdpitch.
 22. A surface acoustic wave filter of claim 21, wherein ametalization ratio of the first IDT electrode, a metalization ratio ofthe second IDT electrode, a metalization ratio of the third IDTelectrode, a metalization ratio of the fourth IDT electrode, and ametalization ratio of the fifth IDT electrode are substantially eaual toeach other.
 23. A surface acoustic wave filter comprising: apiezoelectric substrate; a first filter track having a) at least aninput IDT electrode arranged on said piezoelectric substrate, and b) atleast an output IDT electrode arranged on said piezoelectric substrate,a second filter track having a) at least an input IDT electrode arrangedon said piezoelectric substrate, and b) at least an output IDT electrodearranged on said piezoelectric substrate, wherein each of said first andsecond filter tracks are longitudinally coupled and includes input andoutput IDT electrodes disposed within a respective propagation path inwhich a surface acoustic wave propagates; and each input IDT having themajority of electrode fingers spaced to define a first main pitch, eachoutput IDT having the majority of electrode fingers spaced to define asecond main pitch, within each of said first and second filter tracks,each first main pitch is different in value from each second main pitch,and said first and second filter tracks are substantially identical inconfiguration to each other, and function as one filter in cooperationwith each other.
 24. A surface acoustic wave filter comprising: apiezoelectric substrate; at least an input IDT electrode arranged onsaid piezoelectric substrate; and at least an output IDT electrodearranged on said piezoelectric substrate, wherein said surface acousticwave filter is a longitudinally coupled mode surface acoustic filter andincludes said input IDT electrode and said output IDT electrode disposedwithin a single propagation path in which a surface acoustic wavepropagates, said input IDT electrode has a plurality of electrodefingers pitches; of the electrode fingers pitches of said input IDTelectrode, an electrode fingers pitch having the most pairs of electrodefingers is defined as a main input pitch, said output IDT electrode hasa plurality of electrode fingers pitches, of the electrode fingerspitches of said output IDT electrode, an electrode fingers pitch havingthe most pairs of electrode fingers is defined as a main output pitch,and (a) a peak frequency of a radiation characteristic of the input IDTelectrode having the main input pitch and a peak frequency of aradiation characteristic of the output IDT electrode having the mainoutput pitch are substantially equal, or (b) a peak frequency of aradiation characteristic of the input IDT electrode and a peak frequencyof a radiation characteristic of the output IDT electrode aresubstantially equal.
 25. A surface acoustic wave filter comprising: apiezoelectric substrate; a first filter track having (a) at least aninput IDT electrode arranged on said piezoelectric substrate, and (b) atleast an output IDT electrode arranged on said piezoelectric substrate,a second filter track having (a) at least an input IDT electrodearranged on said piezoelectric substrate, and (b) at least an output IDTelectrode arranged on said piezoelectric substrate, wherein each of saidfirst and second filter tracks are longitudinally coupled and includesinput and output IDT electrodes disposed within a respective propagationpath in which a surface acoustic wave propagates, said input IDTelectrode has a plurality of electrode fingers pitches; of the electrodefingers pitches of said input IDT electrode, an electrode fingers pitchhaving the most pairs of electrode fingers is defined as a main inputpitch, said output IDT electrode has a plurality of electrode fingerspitches, of the electrode fingers pitches of said output IDT electrode,an electrode fingers pitch having the most pairs of electrode fingers isdefined as a main output pitch, and in each of said first and secondfilter tracks, the main pitch of electrode fingers of said input IDTelectrode and the main output pitch of electrode fingers of said outputIDT electrode are different from each other.